This invention relates to a method of improving process control in the manufacture and purification of acetic acid, and a method of manufacturing acetic acid utilizing improved process control.
The prevailing method of acetic acid production involves continuously reacting methanol and carbon monoxide in a stirred reactor. The reaction mixture contains a soluble catalyst from Group 9, specifically iridium or rhodium, and methyl iodide/hydrogen iodide promoters which accelerate the rate of reaction. The two primary reactions which occur in the acetic acid process involve the carbonylation of methanol with carbon monoxide to form acetic acid and a water gas shift reaction which forms carbon dioxide and hydrogen from carbon monoxide and water. Hydrogen production from the water gas shift reaction further leads to the formation of a propionic acid impurity in the reactor solution.
A complex network of dependent equilibria involving liquid acetic acid reaction components exists in the reactor. Even slight changes in these equilibria can induce considerable and adverse effects on catalyst stability and activity in the reactor. These changes can ultimately lead to compositional changes in liquid streams entering the purification section of an acetic acid plant practicing methanol carbonylation technology.
The use of on-line infrared analysis in controlling reactor liquid composition has been described in U.S. Pat. No. 6,103,934 entitled MANUFACTURING AND PROCESS CONTROL METHODS, and U.S. patent application Ser. No. 09/611,067 filed Jul. 6, 2000 and entitled MANUFACTURING AND PROCESS CONTROL METHODS, each incorporated herein by reference in their entirety. Real time analysis of reactor solution allows instant adjustments to be made via process control loops to effect optimal reactor performance. In the process of acetic acid production and purification, it is of course necessary to remove other components from the acetic acid products and, where necessary, return these other components via recycle loops to the reactor or other parts of the process. The composition of these purification/recycle streams is partially a function of reactor composition/performance and partially a function of recycle/purification column performance.
There is thus a need to implement process control via on-line infrared analysis to the purification and recycling section of an acetic acid reaction system, such as in a manufacturing plant.
The present invention provides a method of real time process control of component concentrations in a reaction system for the production of acetic acid from the carbonylation of methanol. To this end, and in accordance with the present invention, samples of reaction system solution are collected from columns and/or transfer lines downstream of a reactor vessel, and the concentration of one or more components in the sample is measured by an infrared analyzer. The concentration measurements are used to make adjustments in the process. The concentration of one or more components is adjusted, either directly or indirectly, in one or more locations in the reaction system in response to the downstream measurements. For example, the flow rate of a solution stream in a transfer line can be increased or decreased going into or out of a column to alter the concentration of one or more of the components in that column or another vessel in the reaction system. Alternatively, the temperature of the solution in a column or stream or the temperature profile or gradient in a column could be increased or decreased to affect the concentration of one or more components in the reaction system solution. Also, the concentration of a reaction system component can be adjusted directly by direct addition or extraction of that component into or out of the solution. For example, water concentration in the reaction system can be adjusted directly by increasing or decreasing the water feed into the reactor vessel, and indirectly by increasing or decreasing recycle streams containing water to the reaction section. The vent gas rate out of the reactor vessel or a column can also be increased or decreased. Thus, reaction system component concentrations can be adjusted directly or indirectly by varying any number of process variables in the reaction system. Further, adjustment in one location of the reaction system may cause concentration changes at either that location or upstream or downstream of that location. For optimum process control, the measurements are transmitted to a control unit for real time analysis, and the adjustments are made substantially instantly after the infrared analysis. There is thus provided a method for continuously updating the conditions of the reaction system to enhance process control in real time of the overall process to thereby optimize the production and purification of acetic acid product.